Processing materials, such as biological materials or chemical materials often requires the mixing of these materials within a container. The container can be a test tube or beaker, for example. A rack that supports multiple containers is sometimes used when mixing batches of materials. Mixing can be achieved by shaking the container or by using a stirring rod or impeller immersed in the material. Some mixers use a coated magnet placed inside the container. The coated magnet is magnetically driven in a rotary motion to mix the contents of the container. Non-invasive mixers, such as shakers, can be advantageous because they do not introduce stirrers, mixing blades, or other mechanical devices into direct contact with the materials to be mixed, thus avoiding potential contamination of those materials by the blades or other mechanical devices.